1. Field of the Invention
The present invention relates to an optical scanning device for use in digital copying machines, laser printers, or laser facsimile machines and image forming apparatuses.
2. Description of the Related Art
Optical scanning devices are incorporated in laser printers. A typical optical scanning device is configured so that a light beam emitted from a light source device is deflected by an optical deflector, and the deflected light beam is converged onto a scanning surface by a scanning and image optical system, such as an Fθ lens, in the form of a beam spot. The scanning surface is then optically scanned with the beam spot (this scanning process is called “main scanning”). The scanning surface is typically photosensitive surface of a photosensitive member.
A full-color image forming apparatus typically includes four photosensitive members, a plurality of light source devices, one deflecting device, and four optical systems. The four photosensitive members are arranged along the conveyance direction of a printing paper. Light flux of light beams that are emitted from the light source devices are deflected at the deflecting device and directed on the photosensitive members at the optical systems thereby forming beam spots on the photosensitive members. The photosensitive members are then scanned with these beam spots thereby forming latent images on the photosensitive members. The latent images are then developed into visible images at developing devices that use developers in mutually different colors, namely in yellow, magenta, cyan, and black, for example. Subsequently, the visible images are sequentially transferred and fixed onto one printing paper on top of one another, so that a color image is obtained.
An image forming apparatus that includes two or more photosensitive members that are arranged along the conveyance direction of a printing paper is known as a tandem-type image forming apparatus. In the tandem-type image forming apparatuses, a single optical deflector is used in common among all the photosensitive members.
For instance, Japanese Patent Application Laid-open No. H9-54263 discloses a technique by which a plurality of light fluxes that are substantially parallel to one another and are apart from one another in a sub-scanning direction are made incident to an optical deflector, whereas a plurality of scanning optical devices that correspond to the light fluxes are arranged in the sub-scanning direction so that a scanning process is performed. Further, Japanese Patent Application Laid-open No. 2001-4948, Japanese Patent Application Laid-open No. 2001-10107, and Japanese Patent Application Laid-open No. 2001-33720 each disclose a technique by which light fluxes are made incident to an optical deflector from one side of the optical deflector, while scanning optical systems that are configured so as to be in three layers are used. A plurality of light fluxes that travel toward mutually different scanning surfaces pass through a first and a second scanning optical system layers, whereas a third scanning optical system layer is provided for each of the mutually different scanning surfaces. By employing a single optical deflector in common among the plurality of scanning surfaces, it is possible to reduce the number of components and to make the image forming apparatus compact.
A polygon mirror is typically used as the optical deflector. If a single polygon mirror is used corresponding to a plurality of photosensitive members, although it is possible to reduce the number of components, there is a drawback that the polygon mirror needs to be made larger in the sub-scanning direction. This is because, it is necessary to cause the light beams deflected at the optical deflector to be incident onto the photosensitive members are arranged in a row in the sub-scanning direction and substantially parallel to each other. Polygon mirror, in general, and larger polygon mirrors, in particular, are expensive.
Japanese Patent Application Laid-open No. 2003-5114 discloses, as an attempt to reduce the costs, an oblique incident optical system with which a light beam is made incident onto the deflecting reflection surface of a single optical deflector at an angle in the sub-scanning direction. In the oblique incident optical system, after being deflected and reflected on the deflecting reflection surface, each of a plurality of light beams that have been separated by a turn-back mirror or the like is introduced to a corresponding one of photosensitive members serving as scanning surfaces. In this configuration, the angle, in the sub-scanning direction, of each of the light beams (i.e., the angle at which each of the light beams is obliquely incident to the optical deflector) is set at such an angle that the light fluxes can be separated by the mirror. With the oblique incident optical system it is possible to keep the distance small between the light beams that are positioned adjacent to one another in the sub-scanning direction, while the mirror is able to separate the light fluxes from one another, without having to use a large optical deflector (i.e., without having to configure a polygon mirror so as to have too many layers or to be too thick in the sub-scanning direction).
However, the oblique incident optical system has the problem of having a large bending of a scanning line. The amount of occurrence of the bending of the scanning line varies depending on the oblique incident angle, in the sub-scanning direction, of each of the light beams. If a bending of a scanning line occurs, when latent images that have been drawn by the light beams are made visible and are overlapped on top of one another by using toners for different colors, the images will exhibit a color registration error. Also, in the oblique incident optical system, because the light flux is incident while being distorted with respect to a scanning lens, the wave aberration increases, and the level of optical performance is significantly degraded especially for the periphery image height. Thus, the beam spot size becomes large, and it could be one of the causes that hinder the endeavor to make high quality images.
Some methods have been proposed to correct the large bending of a scanning line, which is a problem peculiar to the oblique incident optical system described above. As an example, Japanese Patent Application Laid-open No. H11-14932 discloses a method by which a scanning and image forming optical system is configured so as to include a lens that has a lens surface of which the unique inclination in the sub-scanning cross-sectioned plane is altered toward the main-scanning direction, so that the bending of the scanning line can be corrected. As another example, Japanese Patent Application Laid-open No. H11-38348 discloses a method by which a scanning and image forming optical system is configured so as to include a correcting reflection surface that has a reflection surface of which the unique inclination in the sub-scanning cross-sectioned plane is altered toward the main-scanning direction, so that the bending of the scanning line can be corrected. Further, Japanese Patent Application Laid-open No. 2004-70109 discloses yet another method by which a light flux that is obliquely incident is caused to pass on the outside of the axis of a scanning lens, so that the positions of the scanning lines are brought into alignment by using a surface by which the amount of asphericity of the non-generatrix of the scanning lens changes along the main-scanning direction. Japanese Patent Application Laid-open No. 2004-70109 discloses an example in which a correction process is performed by using one scanning lens. By using this method, it is possible to correct the bending of the scanning line like the one described above; however, this publication does not disclose any technique related to degradation of a beam spot diameter due to an increase in the wave aberration, which is explained below.
As another example, Japanese Patent Application Laid-open No. H10-73778 discloses a technique to realize an optical scanning device that is able to properly correct the bending of the scanning line and degradation of the wave aberration, which are the problems related to the oblique incident method as described above. According to this technique, the scanning and image forming optical system includes a plurality of rotating asymmetric lenses, and the shape of a generatrix that connects the vertices of the non-generatrix on the lens surface of the rotating asymmetric lenses is configured to be curved in the sub-scanning direction. This publication discloses the correcting method based on design values. In actuality, however, the bending of the scanning line and the wave aberration that are corrected at the designing stage are degraded due to a change in the oblique incident angle caused by the influence of assembly errors, process errors, or the like of the optical elements. As a result, a problem still remains where the quality of the image is lowered.
Another problem is that, in the case where the oblique incident angle changes due to the influence of assembly errors, process errors, or the like, the optical elements such as the scanning lens and the turn-back mirror are not able to cause the light beams to pass the positions as designed. Thus, a problem arises where the beam spot size may vary, and moreover, the light beams do not reach the scanning surfaces. This problem may be alleviated by processing and assembling the optical elements with a higher level of precision. However, this solution is not realistic because the costs of the components will greatly increase, and longer time will be required to assemble the optical elements due to the complicated assembly process.
In an oblique incident optical system, when the angle at which the light beam is obliquely incident to a deflecting reflection surface increases, various types of aberrations are degraded, and also the level of optical performance is degraded. More specifically, due to the degradation of the wave aberration, the beam spot diameters are degraded, and the degree of the bending of the scanning line also increases. In view of the level of optical performance and making the optical scanning device compact, it is preferable to make the oblique incident angle as small as possible. However, when the oblique incident angle is small, it becomes difficult to separate the light beams for each of the corresponding scanning surfaces. The reason for this can be explained as follows: To separate the light beams travelling toward the mutually different scanning surfaces while arranging the oblique incident angle to be small, the distance between the light beams in the sub-scanning direction at the position of separation is configured so as to be as small as possible. In this situation, if the oblique incident angle is changed in such a manner that the positions in which the light beams pass change in the sub-scanning direction, the light beams do not pass the positions that they are supposed to pass according to the designs of the optical elements. As a result, the beam spot diameters become large. Consequently, depending on whether the light beams pass the positions as designed or how much the light beams deviate from the positions according to the design, the beam spot diameters fluctuate. Thus, it is not possible to obtain stable beam spot diameters.
Japanese Patent Application Laid-open No. 2004-271906 discloses a light source device including an oblique incident optical system. However, the problem described above, that is, the oblique incident angle is changed due to the influence of assembly errors, process errors, or the like, is not solved. Alternatively, another method may be possible by which, after all the optical elements are assembled together, the coupling lens is adjusted so that the angle in the sub-scanning direction can be adjusted. However, when this method is used, the assembly and the adjustment of the light source device become complicated. Thus, this method is not very desirable because it leads to another problem where the time required to make the adjustments increases.